Now I'd like to talk about phase diagrams. In order to understand stainless steel, we need to know about the phase diagrams a little. And the base phase diagram in stainless steel is the iron-chromium binary phase diagram. In the phase diagram, you can understand that at specific temperature and chemical composition, we can predict the phases the alloy can contain. Firstly, in this iron-chromium binary phase diagram, some important features I’d like to talk about are these. Firstly, the sigma phase here. The sigma phase, which is a very brittle and harmful phase in stainless steel, is formed within these temperature ranges, for example from around 500 to 800 degrees Celsius. So, avoiding this sigma phase is very important. And also, this sigma phase is decomposed into an iron rich phase and chromium rich alpha prime phase. The alpha prime phase also is very brittle. This is also what we need to avoid. And since this decomposition usually occurs at around 475 degrees Celsius after a very long time, we call this phenomena “475°C embrittlement”. And as I said before, this alpha prime phase adversely influences the mechanical properties, so we must control when we make the stainless steel by heat treatment. And here in this phase diagram, if the chromium content is higher than 12.7 percent, we can obtain around 100% of ferritic (alpha phase) stainless steel. And if the chromium content is lower than around 12 percent, then sometimes we can obtain 100% austenite and sometimes we can obtain both phases. So depending on this chromium content and also the temperature, we can control the phases which the material can contain. The second phase diagram here I show is the ferritic and martensitic stainless steel. The left figure shows the base iron and 13 percent of chromium containing alloy, and if we change the carbon content, then we can obtain this left-side phase diagram. And the right phase diagram is medium chromium grade 17 percent which contains 17 percent chromium, and as a function of carbon and temperature, we can obtain this phase diagram. So in the left phase diagram, if you see this one, you can see if the carbon content is very low, like in these ranges, we can keep the ferritic phase, delta or alpha phase up to room temperature. That's why we can obtain ferritic stainless steel. However, if the steel contains higher than 0.1 percent, then the steel transforms to austenite or contains austenite. And by quenching this austenite to room temperature by rapid cooling, we can obtain martensite. So the ferritic stainless steel and martensitic stainless steel is very close. However, the big difference is the amount of carbon. So the amount of carbon determines ferritic or martensitic stainless steel. Once again, generally the martensitic grade contains higher amount of carbon and usually more than 0.1 percent of carbon. And by this amount of carbon, we can obtain hundred percent of austenite, here and here, and by quenching this austenite, we can obtain martensitic stainless steel. And as you can see, as you can compare these left side and the right side phase diagrams, if the chromium content is lower, then we can obtain a very wide range of 100 percent of austenite rather than a small range of 100 percent of austenite in higher chromium content. So by controlling the chromium content and also the carbon content, we can make sometimes ferritic grade of stainless steel and sometimes martensitic grade of stainless steel. Now I want to talk about the austenitic grade and duplex grade stainless steels. The most popular austenitic stainless steel is AISI 304 stainless steel, which contains around 18 percent chromium and around 8 to 12 percent of nickel. So using this chemical composition, we can make 100 percent austenite. And the typical phase diagram, as you can see in this diagram, this is based on iron, 18 percent chromium and the x-axis is changing nickel. So by changing this nickel as a function of temperature, we can obtain 100% austenite if the nickel content is higher than 8%. By using this chemical composition, that means, by the addition of 8% or 12% nickel into iron and 18% chromium steel, we can keep this 100 percent austenite at room temperature. That's why we can make 100 percent austenitic stainless steel. And in duplex stainless steel, usually we control the chromium content and nickel content as shown in this figure, and the number here 25 usually means chromium content or 7 means nickel content. Here 2507 means 25 percent chromium and 7 percent nickel steel, and the typical duplex stainless steel which is called 2205 has 22 percent chromium and 5 percent nickel. So by changing this chemical composition, we can obtain a wide range of duplex stainless steel. And here in this phase diagram, you can see this range, the left side has 100% austenite and the right side has 100% ferrite. And within these ranges, we can obtain around 50 percent austenite and around 50 percent ferrite phase which is so-called duplex stainless steel. The important features in these duplex stainless steels – usually duplex stainless steel solidifies from liquid phase to solid phase by formation of delta ferrite solidification. And this solid phase transforms to delta plus gamma. So that means some portion of delta phase transforms to gamma. And here also within these temperature ranges between 800 to 850 degrees Celsius sigma phase is formed, and since this sigma phase is very harmful in mechanical properties and also in corrosion properties, we have to avoid these sigma phase formations. And some important microstructural changes in stainless steel are written here. Because stainless steel contains a lot of alloying elements, there are many reactions between these alloying elements. And all these phases and a few more phases are called second phases which are formed during the heat treatment in stainless steel. And among them, M23C6 carbide is the most important second phase in stainless steels because this is also very harmful to the mechanical properties and also the corrosion properties. And sigma phase, chi phase, eta phase, and nitride, and some other carbide precipitation, and the alpha prime phase which I already talked about, and also some partitioning of alloy elements into duplex stainless steel – all these are important microstructural changes which we can come across in stainless steel. This diagram or this figure shows just one example of precipitates in austenitic stainless steel. This stainless steel contains 22 percent chromium, 21 percent nickel, and some molybdenum and nitrogen. You can see a lot of different second phases which can appear from just one condition of chemical composition and heat treatment. This is the sigma phase. This is the chi phase. And this is M23C6 carbide which has a different shape. And this is chromium nitride, and sometimes even aluminum nitride can be formed in one condition of just one alloy composition. Duplex stainless steel has also many second phases depending on the temperature and depending on the chemical composition. So this diagram shows if the duplex stainless steel contains higher amount of molybdenum, tungsten, and silicon, then this graph is changing to higher temperature. And if more chromium, molybdenum, tungsten, and silicon is added, the reaction speed of these second phases becomes faster and faster. And in a low temperature range, chromium, molybdenum, copper, and tungsten also makes a faster reaction; that means all of these alloying elements will make faster reaction[s] of second phases. At higher temperature ranges and in the low temperature ranges, different kinds of second phases can be formed. As I said before, the M23C6 carbide is the most important second phase in all kinds of stainless steel. So, we need to avoid this kind of carbide formation. And here in this diagram, the left side diagram, you can see if the carbon content is higher, for example 700 ppm and this is 400 ppm, so if the carbon content is higher, then the reaction speed is very fast. That means in low carbon stainless steel, the reaction is slower. And also, if the grain size is larger, then we can reduce the reaction speed. Although these carbide formations occur in every kind of stainless steel, it is a little bit slower in austenitic stainless steel. And there is a word called sensitization which is a very important phenomenon in stainless steel. That means the carbide formation during the heat treatment. It is called carbide formation along the grain boundary. And due to this carbide formation along the grain boundary, this one causes intergranular corrosion in certain environments. Intergranular means the corrosion occurs along grain boundaries. This is called intergranular corrosion, and as you can see, if the carbide formation occurs along these grain boundaries, then it is the main cause of the sensitization.
